Method of producing bismuth layered compound

ABSTRACT

A process is disclosed for making a bismuth layered compound, such as Bi 2  SrTa 2  O 9 , by forming mixture of Bi 2  O 3 , Ta 2  O 5  and a compound selected from strontium hydroxide or strontium nitrate, grinding the mixture, shaping the ground mixture at elevated temperature and pressure to form a pellet of bismuth strontium tantalum oxide having a fluorite structure and then heating the pellet in a flow of oxygen at 800-1000° C. until a single phase bismuth layered compound is obtained.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method of producing a bismuth layeredcompound for use in an electronic device such as a ferroelectric memory,for example.

2. Description of the Related Art

Bismuth layered compounds including bismuth superconducting oxideshaving a critical temperature of 110 K and ferroelectric memorymaterials play a very important role in industrial applications. Forapplying such bismuth layered compounds to electronic devices, it isnecessary to develop a thin-film fabrication process for those bismuthlayered compounds.

The structure of a bismuth layered compound, e.g., Bi₂ PbNb₂ O₉, is of apseudo tetragonal system having a columnar shape extending in thedirection of a c-axis. It has a repetitive structure comprising layersof a bismuth oxide and oxides of other elements which are laminated in acertain sequence (see G. A. SMOLENSKII et al., SOVIET PHYSICS--SOLIDSTATE, p. 651-655 (1961) and E. C. SUBBARAO, J. Phys. Chem. SolidsPergamon Press. Vol. 23, p. 665-676 (1972)).

In the repetitive structure, the number of bismuth oxide layers in oneunit structure and the length of the unit structure vary from bismuthlayered compound to bismuth layered compound.

Attempts to apply such bismuth layered compounds to electronic deviceshave been made in the art. According to one of the efforts, a thin filmof a bismuth layered compound which exhibits good ferroelectricproperties is produced by a spin coating process such as a MOD (MetalOrganic Deposition) process.

However, the spin coating process fails to meet requirements for cleanenvironments that are to be established in the actual semiconductorfabrication process.

For the above reasons, there has been a need for a new thin-filmfabrication process for manufacturing a thin film of a bismuth layeredcompound. To make a thin film of an oxide, however, it is difficult torely on oxidation carried out by an ultrahigh vacuum process such asmolecular beam epitaxy or laser ablation. The application of the MOCVD(Metal Organic Chemical Vapor Deposition) process which is widely usedfor semiconductor fabrication has encountered obstacles because ahydrogen gas cannot be used as a carrier and a good source material isnot available.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide a methodof producing a bismuth layered compound which has good crystallineproperties and good electric properties.

According to the present invention, there is provided a method ofproducing a bismuth layered compound comprising the step of heating acompound of fluorite structure as a precursor into a bismuth layeredcompound.

The bismuth layered compound thus produced has good crystallineproperties and good electric properties.

Specifically, a compound of fluorite structure as a precursor isproduced by a deposition process, and the precursor is heated by aoxidizing and crystallizing process, i.e., a hot annealing process, inan oxidizing atmosphere.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic perspective view of the crystalline structure of ametal oxide of a typical fluorite structure;

FIG. 2 is a diagram showing an X-ray diffraction pattern of a thin filmcompound of fluorite structure according to the present invention;

FIG. 3 is a diagram of an electron diffraction image of the thin film ofthe compound of fluorite structure according to the present invention;

FIG. 4 is a diagram showing an X-ray diffraction pattern of a bismuthlayered compound which is produced by annealing;

FIG. 5 is a diagram of a hysteresis curve of the bismuth layeredcompound; and

FIG. 6 is a graph showing the relationship between applied voltages andγ values in the process of plotting the hysteresis curve shown in FIG.5.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

According to the present invention, a compound having a fluoritestructure is produced as a precursor, and the precursor is then heatedinto a desired bismuth layered compound.

A crystalline structure of fluorspar (CaF₂) has long been known asexhibiting a typical fluorite structure. The crystalline structure offluorspar is of a cubic system, and contains calcium atoms positioned attetrahedral sites and fluorine atoms at cubic sites.

The fluorite structure is found in not only fluorspar, but also manyfluorides (MF₂) and oxides (MO₂) of various elements including Ce(cerium), Pr (praseodymium), Tb (terbium), and actinoids (Th(thorium)˜Am (americium)).

Because these compounds are nonstoichiometric compounds, they aretolerant of large compositional changes at the sites of both metal andanion (F or O) elements.

For example, ZrO₂ (zirconium oxide) allows a large amount of alkalineearth metal or another element to be contained as a solid solution, anduranium oxide can be a nonstoichiometric compound with excessive oxygen.

The fluorite structure of the above oxides is of a cubic system and hashigh spatial symmetry, as with fluorspar, with metal atoms positioned atthe sites of calcium atoms of fluorspar and oxygen atoms at the sites offluorine atoms of fluorspar. The crystalline structure of an oxide (BO₂)of fluorite structure is shown in FIG. 1. As shown in FIG. 1, metalatoms (B) are of a face-centered cubic lattice whereas oxygen atoms (O)are of a unit cubic lattice. It can also be seen that a metal atom (B)is positioned at the center of a cube composed of oxygen atoms (O), andan oxygen atom (O) is positioned at the center of a regular tetrahedroncomposed of metal atoms (B).

The fluorite structure of a bismuth compound, which is a precursoraccording to the present invention, is basically of the crystallinestructure shown in FIG. 1, and contains bismuth and other metal atomsplaced at the sites of B and oxygen atoms placed at the sites of oxygenatoms.

Due to the above compositional or other changes, the fluorite structureof the precursor actually has its symmetry slightly poorer than thecubic system of a typical fluorite structure, and may be of a structureslightly changed from a typical fluorite structure.

Various existing film fabrication processes may be used to form acompound, especially a thin film thereof, having a fluorite structurewhich will serve as a precursor. These film fabrication processesinclude a sol-gel process, a spin coating process such as a MOD process,an LSMCD (Liquid Source Misted Chemical Decomposition) process, a CVD(Chemical vapor Deposition) process using a metal halide, a MOCVD (MetalOrganic Chemical Vapor Deposition) process using DPM(DiPivaloylMehanato) or the like as a source, a flash CVD process inwhich a source is carried and mixed in a liquid phase and evaporated byrapid depressurization in a reactive chamber, and physical evaporationprocesses including a vacuum evaporation process, a molecular beamevaporation process, a laser ablation process, and a sputtering process,etc.

According to the spin coating process, for example, a compound offluorite structure can be produced at about 700° C. According to the CVDprocess, a compound of fluorite structure can be produced at a lowertemperature of at most 600° C.

In a method of producing a bismuth layered compound according to thepresent invention, a thin film as a precursor has a composition which isthe same as or close to a bismuth layered compound desired to beproduced.

The bismuth layered compound desired to be produced may be, for example,Bi₂ (Sr,Ba,Ca)(Ta,Nb)₂ O₉ which is used as a ferroelectric material, ora Bi oxide superconducting material, or the like.

The thin film of a compound of fluorite structure which serves as aprecursor is grown preferably under the following conditions:

Reaction temperature: 400˜700° C.;

Reaction gas pressure: 0.1˜50 torr;

Carrier gas: an oxidizing gas containing at least 5% of oxygen.

The thin film of a compound of fluorite structure which serves as aprecursor is annealed preferably under the following conditions:

Annealing temperature: ranging from the film growth temperatureinclusive to 850° C. inclusive;

Atmosphere: oxidizing atmosphere.

The desired bismuth layered compound can be produced under the aboveconditions.

The growth of a thin film of a bismuth layered compound whosecomposition is Bi₂ SrTa₂ O₉ will be described below.

If the flash CVD process is used, among other processes described above,then it is possible to synthesize a precursor of fluorite structurewhich is grown as deposited, rather than being annealed. The growth of athin film of a bismuth layered compound according to the flash CVDprocess will be described below.

A substrate of silicon with layers of Ti, Pt deposited successivelythereon by sputtering is used.

CVD sources may be a Bi source of BiPh₃ (triphenyl bismuth), Bi(O-Tol)₃,or the like, an Sr source of Sr(DPM)₂ (dipivaloyl strontium), Sr(Me₅C₅)₂.2THF, or the like, and a Ta source of Ta(OCH₃)₅, Ta(O-iPr)₅, or thelike.

If the Bi source is of a material containing oxygen, such as Bi(O-iPr)₅,Bi(O-tC₄ H₉)₃, Bi(O-tC₅ H₁₁)₃, Bi(O-Tol)₃, or the like, then a compoundof fluorite structure may be produced even in the absence of theoxidizing atmosphere.

The above source materials are dissolved in an organic solvent or thelike, and carried in a liquid phase, together with an Ar (argon) carriergas, into a reactor. The reactor is rapidly depressurized preferably to0.1˜10 torr to evaporate the source solution, thus depositing the sourcematerial in a gas phase on the substrate. If pure oxygen is used as anoxidizing gas, then it is supplied at a rate adjusted such that thepartial pressure of oxygen is about 50% of the total pressure.

The source materials are mixed at a ratio which is established properlypreferably by analyzing the composition of a thin film produced on atrial basis by EPMA (Electron Probe Microanalyzer) or an X-rayfluorescence spectroscopy, comparing the analyzed composition with adesired composition, and adjusting the proportions of the sourcematerials.

If the source materials are supplied at a correct composition ratio,then it is possible to obtain a thin film of fluorite structure in asingle phase or an almost single phase at a substrate temperatureranging from 400 to 700° C.

The precursor thus produced is heated preferably at 800° C. for one hourin an oxidizing atmosphere, preferably in a flow of oxygen under normalpressure, thereby producing a thin film which comprises a main phase ofa bismuth layered compound of Bi₂ SrTa₂ O₉.

If electrodes of platinum are formed on the thin film by sputtering,then it is possible to measure the thin film for electric properties,and observe a clear hysteresis curve thereof.

Substances produced by the CVD process are generally diverse because ofcomplex multistage reactions between the source materials, and subjectto compositional changes due to nucleation in a gas phase, thegeneration of by-products, and re-evaporation from the substance, forexample. For this reason, it is difficult to obtain a thin film in adesired phase as deposited, and when the thin film thus obtained isannealed.

The method according to the present invention is effective to produce abismuth layered compound in a desired phase by heating a precursor offluorite structure as a crystal nucleus.

Bismuth oxides often exhibit optimum electric properties even thoughtheir compositions are slightly different from the theoreticalcomposition. Actually, the thin film of a bismuth layered compoundaccording to the present invention contains a somewhat excessive amountof bismuth.

The compound of fluorite structure as a precursor may be of a cubicsystem or a system which contains slight asymmetries.

A specific example of the method of producing a bismuth layered compoundaccording to the present invention will be described below. In thisexample, a thin film of a bismuth layered compound whose composition isBi₂ SrTa₂ O₉ was fabricated.

Layers of Ti and Pt were successively deposited, each to a thickness of100 nm, on the (100) face of naturally oxidized silicon at roomtemperature by sputtering, thus producing a substrate.

Source materials for use in the CVD process were selected to be BiPh₃,Sr(DPM)₂, and Ta(O-iPr)₅, and dissolved in an organic solvent of THF(tetrahydrofuran) or the like, producing respective solutions eachhaving a concentration of about 0.1 M/l.

The solutions were mixed together at an initial liquid volume ratio ofBi:Sr:Ta=2:1:2, and then carried into a vaporizer. The vaporizer hadbeen heated so that the source materials would not be separated andattached to the inner wall surface of the vaporizer.

The vaporized solution is introduced from the vaporizer, together with acarrier gas of Ar, into a reactor.

The vaporizer and the reactor were depressurized to about 10 torr tovaporize the source solution. The vaporized source solution was thencarried in a gas phase and deposited on the substrate while thesubstrate was kept at a temperature of about 600° C.

An oxidizing gas, such for example as pure oxygen, a balancing gas of amixture of Ar and 20% of oxygen, ozone, N₂ O, NO₂, or the like wasintroduced directly into the reactor, not through the vaporizer.

The carrier gas of Ar and the oxidizing gas were supplied at a ratewhich was adjusted to about 500 sccm by respective mass flowcontrollers.

In this manner, a thin film of fluorite structure in an almost singlephase, which had excellent crystalline properties, was obtained.

The thin film thus produced had an X-ray diffraction pattern as shown inFIG. 2, and an electron diffraction image as shown in FIG. 3.

Peaks indicated by F in FIG. 2 represent a bismuth compound of fluoritestructure, and peaks indicated by Pt in FIG. 2 represent Pt in thesubstrate beneath the thin film. The numerals associated with the peaksF represent crystal faces corresponding to the peaks.

In FIGS. 2 and 3, the peaks and the diffraction image are observed whichare reflected by the (111), (200), (220), (311), (222), (400), (331),(420), (422) faces, well indicating the features of a fluoritestructure.

The thin film made of the compound of fluorite structure was heated at800° C. for one hour in a flow of oxygen under normal pressure.

As a result, there was produced a thin film of a bismuth layeredcompound which had a composition of Bi₂ SrTa₂ O₉ as a main phase.

An X-ray diffraction pattern of the thin film of the bismuth layeredcompound is shown in FIG. 4.

In FIG. 4, Pt indicates peaks representing platinum, and Bi indicatespeaks represents the bismuth layered compound.

A comparison between the X-ray diffraction patterns shown in FIGS. 2 and4 revealed that a phase change occurred due to annealing in theoxidizing atmosphere, changing the positions of the peaks, i.e., thecrystalline structure of the bismuth layered compound. The observeddiffraction image in a limited field of view of the bismuth layeredcompound was in good conformity with calculated results, and confirmedthat the bismuth layered compound of good crystalline structure wasobtained.

For evaluating the characteristics of the thin film fabricated in theabove example, upper electrodes of platinum having a thickness of about200 nm were formed on the thin film by sputtering, and the thin film wasmeasured for hysteresis.

FIG. 5 shows a hysteresis curve of the measured thin film. Thehysteresis curve was plotted while the applied voltage was being changedfrom 1 to 5 V. When the applied voltage was 3 V or higher, a 2 Pr value(which is the difference between a positive remanent polarization Pr⁺and a negative remanent polarization Pr⁻) representing a polarization isof about 10 μC/cm².

FIG. 6 shows the relationship between the magnitudes of the appliedvoltages and the values of γ=2 Pr/(Ps⁺ -Pr⁺) in the process of plottingthe hysteresis curve shown in FIG. 5. Ps⁺ represents the polarization atthe maximum applied voltage. From the viewpoint of the hysteresis curve,the thin film can produce a greater output as the value of γ is larger.It can be seen from FIG. 6 that the thin film should preferably be usedin combination with an applied voltage of about 1 V.

The position of the value of γ, i.e., the operating voltage, can becontrolled by the thickness of the thin film. The effective output of anactual device greatly depends on the operating voltage, the bit linecapacitance, etc.

In the above embodiment, the desired bismuth layered compound has acomposition of Bi₂ SrTa₂ O₉. However, thin films of desired compositionsmay be produced for forming bismuth layered compounds of desiredcompositions.

While the fabrication of a thin film of a bismuth layered compound hasbeen described above in the illustrated embodiment, a bulk bismuthlayered compound may also be fabricated by the method according to thepresent invention. A process of fabricating such a bulk bismuth layeredcompound will be described below.

Bismuth oxide (Bi₂ O₃), strontium hydroxide (Sr(OH)₂) or strontiumnitrate (Sr(NO₃)₂), and tantalum oxide (Ta₂ O₅) are weighed to achievethe composition ratio of a desired bismuth layered compound,sufficiently mixed together, and ground in a mortar into a powderymaterial.

The strontium material is in the form of a hydroxide or nitrate whichcan be decomposed at a relatively low temperature, rather than acarbonate which is usually used.

The powdery material is then shaped into a pellet under pressure, andthe pellet is kept at 700° C. in the atmosphere for three days.

After the pellet is then heated in a furnace, it is quickly removed fromthe furnace, and charged into liquid nitrogen by which it is quenched.

In this manner, a bismuth strontium tantalum oxide of fluorite structurewhich is represented by:

    (Bi, Sr, Ta)O.sub.2±δ

is obtained.

The bismuth strontium tantalum oxide of fluorite structure is heated asa precursor to 800˜1000° C. in a flow of oxygen under normal pressure,thereby producing a bismuth layered compound which is of an almostsingle phase.

With the method according to the present invention, a compound offluorite structure as a precursor is heated in an oxidizing atmosphereto produce a bismuth layered compound having good electric propertieswhich cannot easily be directly produced by the conventional process.

Since the fluorite structure is tolerant of large compositional changesand bismuth layered compounds exhibit optimum electric properties eventhough their compositions are slightly different from the theoreticalcomposition, it is possible to produce a bismuth layered compound ofsuch a composition as to exhibit optimum electric properties through thefabrication of a precursor comprising a compound of fluorite structure.

Therefore, it is possible to fabricate from the bismuth layered compoundthus produced a ferroelectric thin film having good electric properties.

Because of the large tolerance with respect to compositional changes,process margins, i.e., margins for fabricating conditions, can berelatively large for allowing electronic devices to be manufacturedeasily from the bismuth layered compound.

Having described preferred embodiments of the invention with referenceto the accompanying drawings, it is to be understood that the inventionis not limited to those precise embodiments and that various changes andmodifications could be effected by one skilled in the art withoutdeparting from the spirit or scope of the invention as defined in theappended claims.

What is claimed is:
 1. A method for making a bismuth layered compoundcomprising the steps of:forming a mixture of Bi₂ O₃, Ta₂ O₅ and astrontium compound selected from strontium hydroxide or strontiumnitrate; grinding said mixture to provide a powdered mixture; shapingsaid powdered mixture under conditions of elevated temperature andelevated pressure to provide a pellet comprising a bismuth strontiumtantalum oxide having a fluorite structure; and thereafter, heating saidpellet at a temperature of from 800 to 1000° C. in a flow of oxygenuntil a single phase bismuth layered compound is obtained.